docs: Document what to do with a DynamicValue

[lf-]

Currently the DynamicValue docs page is totally empty: https://buck2.build/docs/api/build/DynamicValue/

It should link to the place from which they are obtained: https://buck2.build/docs/api/build/AnalysisActions/#analysisactionsdynamic_output_new As well as the https://buck2.build/docs/rule_authors/dynamic_dependencies/ page

And it should say how to do things with them in bxl. In particular it should probably mention https://buck2.build/docs/api/build/#dynamic_actions.

Looks like https://buck2.build/docs/api/build/dynattrs/DynamicAttrType/ also is blank.

Someone at work RE'd this system already so I have private examples that I can use to write these docs. I will probably do it eventually.

This is probably "good first issue" material (also if anyone is able to give me triage rights on this repo, I would be happy to triage my own bugs; I know I am filing a lot of them).

[Nero5023]

Thank you for pointing out!

These missing docs are on our backlog.

For bxl, we have a how to guide to use it at here https://buck2.build/docs/bxl/how_tos/how_to_run_actions_based_on_the_content_of_artifact/

[lf-]

I'm working on fixing this now. Should have a PR out shortly.